The present invention relates generally to data input devices which are responsive to the pressure of a stylus or the like on a writing surface to provide an electrical signal indicative of the position of the stylus on the writing surface and more particularly to a method and apparatus for detecting when a stylus has been either just taken up from the writing surface or just placed down in contact therewith, so as to minimize the generation of erroneous indications of stylus position during such stylus-up or stylus-down transition periods.
Numerous techniques have been taught in the art for the conversion of a position of a contact point on the surface of a data input device to an electrical signal indicative of this position. When used in conjunction with a computer system, such signals are commonly digitized and input to the computer as digital X and Y Cartesian coordinates which uniquely indicate the specific position of the stylus on the surface of the writing area. These digitized X and Y coordinates may then be used to manipulate a pointing cursor on a CRT display screen, as a means for generating graphic information, or in other applications.
A conventional pressure activated, two-dimensional graphic input terminal is disclosed in U.S. Pat. No. 3,959,585 issued to Mattes, et al. Another example is described in U.S. Pat. No. 3,885,097 to Pobgee. Generally, these data input devices use two uniformly conductive resistive sheets that overlap in a common writing area and are separated by a small air gap or in some other way are maintained in a normally spaced apart relationship. Affixed to opposite edges of these resistive sheets are conductive strips such that when an energizing potential is developed across the electrodes of a given resistive sheet, a voltage gradient is created across the sheet in a direction perpendicular to the electrodes.
In operation, these resistive sheets are time-alternately energized to generate two perpendicular voltage gradients. The opposite non-energized sheet acts as a sensing probe which is brought into contact with the energized sheet at the point of pressure contact of a stylus positioned on the surface of the writing area. Preferably, the resistive sheets are oriented orthogonal to one another so that the voltages sensed or generated alternately on each of the two resistive sheets when these sheets are in a non-energized state define respective X and Y coordinates. These coordinates can then be digitized and used by a computer.
More specifically, when the first resistive sheet is energized to create, for example, a voltage gradient in the X dimension, a voltage indicative of the position of the point of application of stylus pressure along the X axis is created on the second non-energized resistive sheet which is in electrical contact with the energized sheet at the stylus contact point This voltage is then transmitted to an output circuit. Similarly, when the second resistive sheet is energized to create a voltage gradient in the Y dimension, a voltage indicative of the position of the point of application of stylus pressure along the Y axis is created on the first resistive sheet which is now in a non-energized state. This second voltage is also transmitted to an output circuit.
A significant drawback of such prior art input devices is that erroneous contact point artifacts tend to be created when a stylus is either being moved up off of the writing surface or is initially being placed on the writing surface. These are also referred to as the stylus-up and stylus-down transition periods. The erroneous coordinate values created by these artifacts cause erroneous jumps in the position of a cursor on a computer display screen or an erroneous line to be drawn, depending on the specific application for the data input device. These spurious and erroneous contact points may be caused by the inherent time-multiplexed nature of the determination of the X-Y coordinates as described above. They may also result from contact bounce created as an electrical contact is either being made or broken between the two resistive sheets in response to stylus pressure. This is especially prevalent where just an air gap is used to separate the two resistive sheets in the absence of stylus contact.
Where the resistive sheets are separated by a thin sheet of elastomeric material which acts as an electrical insulator that becomes conductive at a selected point upon the application of pressure over a suitable threshold at that point, another problem exists. The resistance of the connection created by this elastomeric sheet is a steep but still finite function of applied pressure, such that the voltages sensed during stylus-up or stylus-down transitions not only reflect the position of the stylus on the writing area but also are modified to some unknown extent by this time varying resistance. Finally, a mutual capacitance Cm exists between the two resistive sheets whose charge is unknown at any given time. This charge is discharged through the above-described time varying resistance path created in the elastomeric sheet.
All of these problems result in unreliable stylus position information during stylus-up and stylus-down transition periods.
The problem of having erroneous indications of stylus position is more serious in certain more recent applications of such data input devices. In such applications, the end point of the movement of the stylus on the writing surface is the most important data point to be accurately obtained and output to a computer. For example, if the data input device is being used to move a cursor on a computer screen, the user desires to move the cursor to a specific point on the screen using this data input device. If the user's act of taking the stylus off of the writing surface of the input device, however, causes an erroneous indication of stylus movement, the cursor will move about on the computer screen in the same erroneous manner. Consequently, the user is frustrated from being able to accurately position the cursor at a desired point on the computer display screen. This is especially a problem where the computer detects the stylus-up transition and uses this to automatically start a program chosen by the position of the cursor on the computer screen. Erroneous processing would result in this situation.
One prior art attempt to improve the reliability of stylus position detection during stylus-up and stylus-down transition periods is disclosed in U.S. Pat. No. 3,959,585. Although the method described in this patent includes means for periodically biasing one of the resistive sheets to an overvoltage, this is done merely to limit the capacitive effect of the mutual capacitance between the two resistive sheets. This can be a problem especially when large resistive sheets are used, since the capacitance between the sheets can be large enough to cause potentials to be created on the resistive sheet acting as a sensor even then the stylus is not in contact with the surface of the writing area. What is not taught, however, in this prior art patent is any means for detecting the actual stylus-up or stylus-down transition period by means of overvoltage biasing of a non-energized resistive sheet and for novel uses of this transition period detection, as described herein according to the present invention.